The move of so much commerce online (nevermind to virual goods) is just starting to play out in the courts. It will be decades before they even handle what is really going on today. None the less it appears to me they have it right in the recent decision in favor of Ebay against a claim by Tiffany that they allowed knock offs to be sold. http://www.law.com/jsp/article.jsp?id=1202447367856&nd_Circuit_Rejects_Claim_EBay_Violated_Tiffanys_Trademark
I find it interesting that clearly as you scale up you have a greater and greater responsibility to police what your users are doing. With a popular site this is pretty hard though. Therefore its ~really~ about showing that you are “doing what you can” and the appearance that you are trying that are more critical than actual results.
Now if they could just stop issuing and upholding all the ridiculous software patents maybe we would have a sounder basis for commerce online.